1. Field of the Invention
The present invention relates to a method for casting such dental metals as titanium designed to make plate frames, clasps, etc. in the dental technical field, which enables high-quality products with neither casting deficiencies nor casting cavities to be cast be melting dental metal ingots by arc discharge.
2. Statement of the Prior Art
In the dental technical field, titanium has been used for CAST plate, clasps, etc., because it is light in weight, has a certain strength and excels in corrosion resistance and bioaffinity.
So far, metal frames, etc. used for, e.g. crowns or porcelain-fused-to-metal crowns have been cast by melting dental metals with the use of suitable techniques such as high-frequency melting and pouring the molten metals in molds, because the dental metals are noble metals relatively difficult to oxidize. Since the above-mentioned titanium has the property of being likely to undergo oxidization, it should be melted in an atmosphere of such an inert gas as argon. To meet such demand, reliance has been placed on a casting technique wherein dental metal ingots are melted by arc discharge and the obtained molten metals are poured in molds.
According to this casting technique, an arc electrode is disposed above in a hermetically sealed melting chamber, and a crucible made of an electrically conductive material is positioned just below the arc electrode. With the arc electrode and the crucible connected to a cathode and an anode, respectively, the dental metal ingot to be cast is first placed on the crucible. After evacuation to vacuum, the melting chamber is then filled with such an inert gas as argon until its internal pressure amounts to a pressure nearly equal to atmospheric pressure. Subsequently, the ingot is melted by arcs occurring from the arc electrode. Finally, the molten metal is poured into a mold chamber through the inlet of a mold located in said mold chamber which is partitioned from said melting chamber by a partition wall provided with a through-hole formed in its portion positioned below said crucible.
When titanium is used with this casting technique, it should be rapidly poured into the mold through its inlet, partly because the melting point of titanium is higher than melting points of common dental metals, and partly because titanium has to be cast with the mold maintained at room temperature as it cools down and solidifys rapidly--due to the fact that it reacts with mold material at such a high temperature as to cast common noble metal alloys. In other words, titanium must be poured under pressure into the mold by increasing pressure of the melting chamber to make a pressure difference between the melting chamber and the mold chamber.
The mold used is constructed from investment materials composed of a binder/aggregate combination. More specifically, the binder and aggregate are kneaded together with water or an exclusive liquid into slurry. After that, a wax model is invested in the slurry and cured at room temperature, followed by burning out wax at about 700.degree. C. in furnace.
Owing to some air permeability available with such investment material, such investment material may be used for casting noble metal alloys forming part of metal frames, etc. used with crowns or porcelain-fused-to-metal crowns. This is because if it is pressurized with a pressure difference made between the melting and mold chambers, then an increase in the in-mold gas pressure is so suppressed due to its air permeability that the molten dental metal can be well poured into the mold.
Among common investment materials, however, there is a gypsum-bonded investment material using gypsum as the binder. At 700.degree. C. or higher, this material increases in air permeability and decreases in heat resistance, since gypsum decomposes thermally at that temperature and so cannot retain its crystal form. For instance, when a high temperature dental metal like titanium is cast with that investment material, it react with each other to evolve gases in an amount which is so large that the gases cannot escape only through the inherent air permeability the investment material has. This would cause an increase in the pressure in the casting region of the mold, making it impossible to pour the predetermined amount of the molten dental metal in the mold. Othewise gases would be entrained in the molten metal, giving rise to cavities. In order to cast a high temperature dental metal, use has thus been made of a phosphate-bonded investment material using a phosphate as a binder. With this phosphate type of a material in which the binder phosphate combines with a metal oxide to form an amorphous product, gas emissions are limited even upon heated at high temperatures, since it undergoes no or little change in state and is of increased heat resistance. However, this leads to another defect that its air permeability is badly limited. For that reason, it has been proposed to use coarse aggregates for increased air permeability, thereby reducing a pressure rise in the casting regions of molds when molten dental metal are poured in them. However, this method makes the surfaces of casting products rough but, nonetheless, fails to obtain sufficient air permeability.
Thus, when no sufficient air permeability is obtained, the internal pressure of the casting region in the mold is too increased to pour the predetermined amount of molten dental metal into the mold. This gives rise to such disadvantages as casting deficiencies and entrainment of gases in the metal melt, resulting in casting cavities.
Arc melting of a dental metal ingot at around atmospheric pressure causes arcs to concentrate on their local points under the influences of magnetic blow, etc., often making its uniform melting impossible and thereby causing the dental metal ingot to be locally heated to high temperatures. While the molten dental metal ingot in its entirety, the molten metal reacts with the crucible material. In order to prevent this, additional special mechanisms for moving the electrodes are needed.
In order to solve the above-mentioned defects of the prior art, the present invention seeks to provide a method for casting dental metals which can produce well-smoothened castings by stable arc melting with no fear of causing casting deficiencies or cavities.